﻿#region License, Terms and Author(s)
//
// LINQBridge
// Copyright (c) 2007-9 Atif Aziz, Joseph Albahari. All rights reserved.
//
//  Author(s):
//
//      Atif Aziz, http://www.raboof.com
//
// This library is free software; you can redistribute it and/or modify it 
// under the terms of the New BSD License, a copy of which should have 
// been delivered along with this distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A 
// PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
#endregion

// $Id: Enumerable.cs 240 2010-10-19 21:49:03Z azizatif $

//namespace System.Linq
//{
//    #region Imports

//    using System;
//    using System.Collections;
//    using System.Collections.Generic;
//    using System.Diagnostics;
//    using LinqBridge;

//    #endregion

//    /// <summary>
//    /// Provides a set of static (Shared in Visual Basic) methods for 
//    /// querying objects that implement <see cref="IEnumerable{T}" />.
//    /// </summary>

//    static partial class Enumerable
//    {
//        /// <summary>
//        /// Returns the input typed as <see cref="IEnumerable{T}"/>.
//        /// </summary>

//        public static IEnumerable<TSource> AsEnumerable<TSource>(IEnumerable<TSource> source)
//        {
//            return source;
//        }

//        /// <summary>
//        /// Returns an empty <see cref="IEnumerable{T}"/> that has the 
//        /// specified type argument.
//        /// </summary>

//        public static IEnumerable<TResult> Empty<TResult>()
//        {
//            return Sequence<TResult>.Empty;
//        }

//        /// <summary>
//        /// Converts the elements of an <see cref="IEnumerable"/> to the 
//        /// specified type.
//        /// </summary>

//        public static IEnumerable<TResult> Cast<TResult>(
//            this IEnumerable source)
//        {
//            CheckNotNull(source, "source");

//            return CastYield<TResult>(source);
//        }

//        private static IEnumerable<TResult> CastYield<TResult>(
//            IEnumerable source)
//        {
//            foreach (var item in source)
//                yield return (TResult)item;
//        }

//        /// <summary>
//        /// Filters the elements of an <see cref="IEnumerable"/> based on a specified type.
//        /// </summary>

//        public static IEnumerable<TResult> OfType<TResult>(
//            this IEnumerable source)
//        {
//            CheckNotNull(source, "source");

//            return OfTypeYield<TResult>(source);
//        }

//        private static IEnumerable<TResult> OfTypeYield<TResult>(
//            IEnumerable source)
//        {
//            foreach (var item in source)
//                if (item is TResult)
//                    yield return (TResult)item;
//        }

//        /// <summary>
//        /// Generates a sequence of integral numbers within a specified range.
//        /// </summary>
//        /// <param name="start">The value of the first integer in the sequence.</param>
//        /// <param name="count">The number of sequential integers to generate.</param>

//        public static IEnumerable<int> Range(int start, int count)
//        {
//            if (count < 0)
//                throw new ArgumentOutOfRangeException("count", count, null);

//            var end = (long)start + count;
//            if (end - 1 >= int.MaxValue)
//                throw new ArgumentOutOfRangeException("count", count, null);

//            return RangeYield(start, end);
//        }

//        private static IEnumerable<int> RangeYield(int start, long end)
//        {
//            for (var i = start; i < end; i++)
//                yield return i;
//        }

//        /// <summary>
//        /// Generates a sequence that contains one repeated value.
//        /// </summary>

//        public static IEnumerable<TResult> Repeat<TResult>(TResult element, int count)
//        {
//            if (count < 0) throw new ArgumentOutOfRangeException("count", count, null);

//            return RepeatYield(element, count);
//        }

//        private static IEnumerable<TResult> RepeatYield<TResult>(TResult element, int count)
//        {
//            for (var i = 0; i < count; i++)
//                yield return element;
//        }

//        /// <summary>
//        /// Filters a sequence of values based on a predicate.
//        /// </summary>

//        public static IEnumerable<TSource> Where<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            CheckNotNull(predicate, "predicate");

//            return source.Where((item, i) => predicate(item));
//        }

//        /// <summary>
//        /// Filters a sequence of values based on a predicate. 
//        /// Each element's index is used in the logic of the predicate function.
//        /// </summary>

//        public static IEnumerable<TSource> Where<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int, bool> predicate)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(predicate, "predicate");

//            return WhereYield(source, predicate);
//        }

//        private static IEnumerable<TSource> WhereYield<TSource>(
//            IEnumerable<TSource> source,
//            Func<TSource, int, bool> predicate)
//        {
//            var i = 0;
//            foreach (var item in source)
//                if (predicate(item, i++))
//                    yield return item;
//        }

//        /// <summary>
//        /// Projects each element of a sequence into a new form.
//        /// </summary>

//        public static IEnumerable<TResult> Select<TSource, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TResult> selector)
//        {
//            CheckNotNull(selector, "selector");

//            return source.Select((item, i) => selector(item));
//        }

//        /// <summary>
//        /// Projects each element of a sequence into a new form by 
//        /// incorporating the element's index.
//        /// </summary>

//        public static IEnumerable<TResult> Select<TSource, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int, TResult> selector)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(selector, "selector");

//            return SelectYield(source, selector);
//        }

//        private static IEnumerable<TResult> SelectYield<TSource, TResult>(
//            IEnumerable<TSource> source,
//            Func<TSource, int, TResult> selector)
//        {
//            var i = 0;
//            foreach (var item in source)
//                yield return selector(item, i++);
//        }

//        /// <summary>
//        /// Projects each element of a sequence to an <see cref="IEnumerable{T}" /> 
//        /// and flattens the resulting sequences into one sequence.
//        /// </summary>

//        public static IEnumerable<TResult> SelectMany<TSource, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, IEnumerable<TResult>> selector)
//        {
//            CheckNotNull(selector, "selector");

//            return source.SelectMany((item, i) => selector(item));
//        }

//        /// <summary>
//        /// Projects each element of a sequence to an <see cref="IEnumerable{T}" />, 
//        /// and flattens the resulting sequences into one sequence. The 
//        /// index of each source element is used in the projected form of 
//        /// that element.
//        /// </summary>

//        public static IEnumerable<TResult> SelectMany<TSource, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int, IEnumerable<TResult>> selector)
//        {
//            CheckNotNull(selector, "selector");

//            return source.SelectMany(selector, (item, subitem) => subitem);
//        }

//        /// <summary>
//        /// Projects each element of a sequence to an <see cref="IEnumerable{T}" />, 
//        /// flattens the resulting sequences into one sequence, and invokes 
//        /// a result selector function on each element therein.
//        /// </summary>

//        public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, IEnumerable<TCollection>> collectionSelector,
//            Func<TSource, TCollection, TResult> resultSelector)
//        {
//            CheckNotNull(collectionSelector, "collectionSelector");

//            return source.SelectMany((item, i) => collectionSelector(item), resultSelector);
//        }

//        /// <summary>
//        /// Projects each element of a sequence to an <see cref="IEnumerable{T}" />, 
//        /// flattens the resulting sequences into one sequence, and invokes 
//        /// a result selector function on each element therein. The index of 
//        /// each source element is used in the intermediate projected form 
//        /// of that element.
//        /// </summary>

//        public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int, IEnumerable<TCollection>> collectionSelector,
//            Func<TSource, TCollection, TResult> resultSelector)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(collectionSelector, "collectionSelector");
//            CheckNotNull(resultSelector, "resultSelector");

//            return SelectManyYield(source, collectionSelector, resultSelector);
//        }

//        private static IEnumerable<TResult> SelectManyYield<TSource, TCollection, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int, IEnumerable<TCollection>> collectionSelector,
//            Func<TSource, TCollection, TResult> resultSelector)
//        {
//            var i = 0;
//            foreach (var item in source)
//                foreach (var subitem in collectionSelector(item, i++))
//                    yield return resultSelector(item, subitem);
//        }

//        /// <summary>
//        /// Returns elements from a sequence as long as a specified condition is true.
//        /// </summary>

//        public static IEnumerable<TSource> TakeWhile<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            CheckNotNull(predicate, "predicate");

//            return source.TakeWhile((item, i) => predicate(item));
//        }

//        /// <summary>
//        /// Returns elements from a sequence as long as a specified condition is true.
//        /// The element's index is used in the logic of the predicate function.
//        /// </summary>

//        public static IEnumerable<TSource> TakeWhile<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int, bool> predicate)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(predicate, "predicate");

//            return TakeWhileYield(source, predicate);
//        }

//        private static IEnumerable<TSource> TakeWhileYield<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int, bool> predicate)
//        {
//            var i = 0;
//            foreach (var item in source)
//                if (predicate(item, i++))
//                    yield return item;
//                else
//                    break;
//        }

//        private static class Futures<T>
//        {
//            public static readonly Func<T> Default = () => default(T);
//            public static readonly Func<T> Undefined = () => { throw new InvalidOperationException(); };
//        }

//        /// <summary>
//        /// Base implementation of First operator.
//        /// </summary>

//        private static TSource FirstImpl<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource> empty)
//        {
//            CheckNotNull(source, "source");
//            Debug.Assert(empty != null);

//            var list = source as IList<TSource>;    // optimized case for lists
//            if (list != null)
//                return list.Count > 0 ? list[0] : empty();

//            using (var e = source.GetEnumerator())  // fallback for enumeration
//                return e.MoveNext() ? e.Current : empty();
//        }

//        /// <summary>
//        /// Returns the first element of a sequence.
//        /// </summary>

//        public static TSource First<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return source.FirstImpl(Futures<TSource>.Undefined);
//        }

//        /// <summary>
//        /// Returns the first element in a sequence that satisfies a specified condition.
//        /// </summary>

//        public static TSource First<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return First(source.Where(predicate));
//        }

//        /// <summary>
//        /// Returns the first element of a sequence, or a default value if 
//        /// the sequence contains no elements.
//        /// </summary>

//        public static TSource FirstOrDefault<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return source.FirstImpl(Futures<TSource>.Default);
//        }

//        /// <summary>
//        /// Returns the first element of the sequence that satisfies a 
//        /// condition or a default value if no such element is found.
//        /// </summary>

//        public static TSource FirstOrDefault<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return FirstOrDefault(source.Where(predicate));
//        }

//        /// <summary>
//        /// Base implementation of Last operator.
//        /// </summary>

//        private static TSource LastImpl<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource> empty)
//        {
//            CheckNotNull(source, "source");

//            var list = source as IList<TSource>;    // optimized case for lists
//            if (list != null)
//                return list.Count > 0 ? list[list.Count - 1] : empty();

//            using (var e = source.GetEnumerator())
//            {
//                if (!e.MoveNext())
//                    return empty();

//                var last = e.Current;
//                while (e.MoveNext())
//                    last = e.Current;

//                return last;
//            }
//        }

//        /// <summary>
//        /// Returns the last element of a sequence.
//        /// </summary>
//        public static TSource Last<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return source.LastImpl(Futures<TSource>.Undefined);
//        }

//        /// <summary>
//        /// Returns the last element of a sequence that satisfies a 
//        /// specified condition.
//        /// </summary>

//        public static TSource Last<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return Last(source.Where(predicate));
//        }

//        /// <summary>
//        /// Returns the last element of a sequence, or a default value if 
//        /// the sequence contains no elements.
//        /// </summary>

//        public static TSource LastOrDefault<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return source.LastImpl(Futures<TSource>.Default);
//        }

//        /// <summary>
//        /// Returns the last element of a sequence that satisfies a 
//        /// condition or a default value if no such element is found.
//        /// </summary>

//        public static TSource LastOrDefault<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return LastOrDefault(source.Where(predicate));
//        }

//        /// <summary>
//        /// Base implementation of Single operator.
//        /// </summary>

//        private static TSource SingleImpl<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource> empty)
//        {
//            CheckNotNull(source, "source");

//            using (var e = source.GetEnumerator())
//            {
//                if (e.MoveNext())
//                {
//                    var single = e.Current;
//                    if (!e.MoveNext())
//                        return single;

//                    throw new InvalidOperationException();
//                }

//                return empty();
//            }
//        }

//        /// <summary>
//        /// Returns the only element of a sequence, and throws an exception 
//        /// if there is not exactly one element in the sequence.
//        /// </summary>

//        public static TSource Single<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return source.SingleImpl(Futures<TSource>.Undefined);
//        }

//        /// <summary>
//        /// Returns the only element of a sequence that satisfies a 
//        /// specified condition, and throws an exception if more than one 
//        /// such element exists.
//        /// </summary>

//        public static TSource Single<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return Single(source.Where(predicate));
//        }

//        /// <summary>
//        /// Returns the only element of a sequence, or a default value if 
//        /// the sequence is empty; this method throws an exception if there 
//        /// is more than one element in the sequence.
//        /// </summary>

//        public static TSource SingleOrDefault<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return source.SingleImpl(Futures<TSource>.Default);
//        }

//        /// <summary>
//        /// Returns the only element of a sequence that satisfies a 
//        /// specified condition or a default value if no such element 
//        /// exists; this method throws an exception if more than one element 
//        /// satisfies the condition.
//        /// </summary>

//        public static TSource SingleOrDefault<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return SingleOrDefault(source.Where(predicate));
//        }

//        /// <summary>
//        /// Returns the element at a specified index in a sequence.
//        /// </summary>

//        public static TSource ElementAt<TSource>(
//            this IEnumerable<TSource> source,
//            int index)
//        {
//            CheckNotNull(source, "source");

//            if (index < 0)
//                throw new ArgumentOutOfRangeException("index", index, null);

//            var list = source as IList<TSource>;
//            if (list != null)
//                return list[index];

//            try
//            {
//                return source.SkipWhile((item, i) => i < index).First();
//            }
//            catch (InvalidOperationException) // if thrown by First
//            {
//                throw new ArgumentOutOfRangeException("index", index, null);
//            }
//        }

//        /// <summary>
//        /// Returns the element at a specified index in a sequence or a 
//        /// default value if the index is out of range.
//        /// </summary>

//        public static TSource ElementAtOrDefault<TSource>(
//            this IEnumerable<TSource> source,
//            int index)
//        {
//            CheckNotNull(source, "source");

//            if (index < 0)
//                return default(TSource);

//            var list = source as IList<TSource>;
//            if (list != null)
//                return index < list.Count ? list[index] : default(TSource);

//            return source.SkipWhile((item, i) => i < index).FirstOrDefault();
//        }

//        /// <summary>
//        /// Inverts the order of the elements in a sequence.
//        /// </summary>

//        public static IEnumerable<TSource> Reverse<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            CheckNotNull(source, "source");

//            return ReverseYield(source);
//        }

//        private static IEnumerable<TSource> ReverseYield<TSource>(IEnumerable<TSource> source)
//        {
//            var stack = new Stack<TSource>();
//            foreach (var item in source)
//                stack.Push(item);

//            foreach (var item in stack)
//                yield return item;
//        }

//        /// <summary>
//        /// Returns a specified number of contiguous elements from the start 
//        /// of a sequence.
//        /// </summary>

//        public static IEnumerable<TSource> Take<TSource>(
//            this IEnumerable<TSource> source,
//            int count)
//        {
//            return source.Where((item, i) => i < count);
//        }

//        /// <summary>
//        /// Bypasses a specified number of elements in a sequence and then 
//        /// returns the remaining elements.
//        /// </summary>

//        public static IEnumerable<TSource> Skip<TSource>(
//            this IEnumerable<TSource> source,
//            int count)
//        {
//            return source.Where((item, i) => i >= count);
//        }

//        /// <summary>
//        /// Bypasses elements in a sequence as long as a specified condition 
//        /// is true and then returns the remaining elements.
//        /// </summary>

//        public static IEnumerable<TSource> SkipWhile<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            CheckNotNull(predicate, "predicate");

//            return source.SkipWhile((item, i) => predicate(item));
//        }

//        /// <summary>
//        /// Bypasses elements in a sequence as long as a specified condition 
//        /// is true and then returns the remaining elements. The element's 
//        /// index is used in the logic of the predicate function.
//        /// </summary>

//        public static IEnumerable<TSource> SkipWhile<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int, bool> predicate)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(predicate, "predicate");

//            return SkipWhileYield(source, predicate);
//        }

//        private static IEnumerable<TSource> SkipWhileYield<TSource>(
//            IEnumerable<TSource> source,
//            Func<TSource, int, bool> predicate)
//        {
//            using (var e = source.GetEnumerator())
//            {
//                for (var i = 0; ; i++)
//                {
//                    if (!e.MoveNext())
//                        yield break;

//                    if (!predicate(e.Current, i))
//                        break;
//                }

//                do { yield return e.Current; } while (e.MoveNext());
//            }
//        }

//        /// <summary>
//        /// Returns the number of elements in a sequence.
//        /// </summary>

//        public static int Count<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            CheckNotNull(source, "source");

//            var collection = source as ICollection;
//            return collection != null
//                 ? collection.Count
//                 : source.Aggregate(0, (count, item) => checked(count + 1));
//        }

//        /// <summary>
//        /// Returns a number that represents how many elements in the 
//        /// specified sequence satisfy a condition.
//        /// </summary>

//        public static int Count<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return Count(source.Where(predicate));
//        }

//        /// <summary>
//        /// Returns an <see cref="Int64"/> that represents the total number 
//        /// of elements in a sequence.
//        /// </summary>

//        public static long LongCount<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            CheckNotNull(source, "source");

//            var array = source as Array;
//            return array != null
//                 ? array.LongLength
//                 : source.Aggregate(0L, (count, item) => count + 1);
//        }

//        /// <summary>
//        /// Returns an <see cref="Int64"/> that represents how many elements 
//        /// in a sequence satisfy a condition.
//        /// </summary>

//        public static long LongCount<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return LongCount(source.Where(predicate));
//        }

//        /// <summary>
//        /// Concatenates two sequences.
//        /// </summary>

//        public static IEnumerable<TSource> Concat<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second)
//        {
//            CheckNotNull(first, "first");
//            CheckNotNull(second, "second");

//            return ConcatYield(first, second);
//        }

//        private static IEnumerable<TSource> ConcatYield<TSource>(
//            IEnumerable<TSource> first,
//            IEnumerable<TSource> second)
//        {
//            foreach (var item in first)
//                yield return item;

//            foreach (var item in second)
//                yield return item;
//        }

//        /// <summary>
//        /// Creates a <see cref="List{T}"/> from an <see cref="IEnumerable{T}"/>.
//        /// </summary>

//        public static List<TSource> ToList<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            CheckNotNull(source, "source");

//            return new List<TSource>(source);
//        }

//        /// <summary>
//        /// Creates an array from an <see cref="IEnumerable{T}"/>.
//        /// </summary>

//        public static TSource[] ToArray<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return source.ToList().ToArray();
//        }

//        /// <summary>
//        /// Returns distinct elements from a sequence by using the default 
//        /// equality comparer to compare values.
//        /// </summary>

//        public static IEnumerable<TSource> Distinct<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return Distinct(source, /* comparer */ null);
//        }

//        /// <summary>
//        /// Returns distinct elements from a sequence by using a specified 
//        /// <see cref="IEqualityComparer{T}"/> to compare values.
//        /// </summary>

//        public static IEnumerable<TSource> Distinct<TSource>(
//            this IEnumerable<TSource> source,
//            IEqualityComparer<TSource> comparer)
//        {
//            CheckNotNull(source, "source");

//            return DistinctYield(source, comparer);
//        }

//        private static IEnumerable<TSource> DistinctYield<TSource>(
//            IEnumerable<TSource> source,
//            IEqualityComparer<TSource> comparer)
//        {
//            var set = new Dictionary<TSource, object>(comparer);
//            var gotNull = false;

//            foreach (var item in source)
//            {
//                if (item == null)
//                {
//                    if (gotNull)
//                        continue;
//                    gotNull = true;
//                }
//                else
//                {
//                    if (set.ContainsKey(item))
//                        continue;
//                    set.Add(item, null);
//                }

//                yield return item;
//            }
//        }

//        /// <summary>
//        /// Creates a <see cref="Lookup{TKey,TElement}" /> from an 
//        /// <see cref="IEnumerable{T}" /> according to a specified key 
//        /// selector function.
//        /// </summary>

//        public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector)
//        {
//            return ToLookup(source, keySelector, e => e, /* comparer */ null);
//        }

//        /// <summary>
//        /// Creates a <see cref="Lookup{TKey,TElement}" /> from an 
//        /// <see cref="IEnumerable{T}" /> according to a specified key 
//        /// selector function and a key comparer.
//        /// </summary>

//        public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            return ToLookup(source, keySelector, e => e, comparer);
//        }

//        /// <summary>
//        /// Creates a <see cref="Lookup{TKey,TElement}" /> from an 
//        /// <see cref="IEnumerable{T}" /> according to specified key 
//        /// and element selector functions.
//        /// </summary>

//        public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TSource, TElement> elementSelector)
//        {
//            return ToLookup(source, keySelector, elementSelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Creates a <see cref="Lookup{TKey,TElement}" /> from an 
//        /// <see cref="IEnumerable{T}" /> according to a specified key 
//        /// selector function, a comparer and an element selector function.
//        /// </summary>

//        public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TSource, TElement> elementSelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(keySelector, "keySelector");
//            CheckNotNull(elementSelector, "elementSelector");

//            var lookup = new Lookup<TKey, TElement>(comparer);

//            foreach (var item in source)
//            {
//                var key = keySelector(item);

//                var grouping = (Grouping<TKey, TElement>)lookup.Find(key);
//                if (grouping == null)
//                {
//                    grouping = new Grouping<TKey, TElement>(key);
//                    lookup.Add(grouping);
//                }

//                grouping.Add(elementSelector(item));
//            }

//            return lookup;
//        }

//        /// <summary>
//        /// Groups the elements of a sequence according to a specified key 
//        /// selector function.
//        /// </summary>

//        public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector)
//        {
//            return GroupBy(source, keySelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Groups the elements of a sequence according to a specified key 
//        /// selector function and compares the keys by using a specified 
//        /// comparer.
//        /// </summary>

//        public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            return GroupBy(source, keySelector, e => e, comparer);
//        }

//        /// <summary>
//        /// Groups the elements of a sequence according to a specified key 
//        /// selector function and projects the elements for each group by 
//        /// using a specified function.
//        /// </summary>

//        public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TSource, TElement> elementSelector)
//        {
//            return GroupBy(source, keySelector, elementSelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Groups the elements of a sequence according to a specified key 
//        /// selector function and creates a result value from each group and 
//        /// its key.
//        /// </summary>

//        public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TSource, TElement> elementSelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(keySelector, "keySelector");
//            CheckNotNull(elementSelector, "elementSelector");

//            return ToLookup(source, keySelector, elementSelector, comparer);
//        }

//        /// <summary>
//        /// Groups the elements of a sequence according to a key selector 
//        /// function. The keys are compared by using a comparer and each 
//        /// group's elements are projected by using a specified function.
//        /// </summary>

//        public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TKey, IEnumerable<TSource>, TResult> resultSelector)
//        {
//            return GroupBy(source, keySelector, resultSelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Groups the elements of a sequence according to a specified key 
//        /// selector function and creates a result value from each group and 
//        /// its key. The elements of each group are projected by using a 
//        /// specified function.
//        /// </summary>

//        public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TKey, IEnumerable<TSource>, TResult> resultSelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(keySelector, "keySelector");
//            CheckNotNull(resultSelector, "resultSelector");

//            return ToLookup(source, keySelector, comparer).Select(g => resultSelector(g.Key, g));
//        }

//        /// <summary>
//        /// Groups the elements of a sequence according to a specified key 
//        /// selector function and creates a result value from each group and 
//        /// its key. The keys are compared by using a specified comparer.
//        /// </summary>

//        public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TSource, TElement> elementSelector,
//            Func<TKey, IEnumerable<TElement>, TResult> resultSelector)
//        {
//            return GroupBy(source, keySelector, elementSelector, resultSelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Groups the elements of a sequence according to a specified key 
//        /// selector function and creates a result value from each group and 
//        /// its key. Key values are compared by using a specified comparer, 
//        /// and the elements of each group are projected by using a 
//        /// specified function.
//        /// </summary>

//        public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TSource, TElement> elementSelector,
//            Func<TKey, IEnumerable<TElement>, TResult> resultSelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(keySelector, "keySelector");
//            CheckNotNull(elementSelector, "elementSelector");
//            CheckNotNull(resultSelector, "resultSelector");

//            return ToLookup(source, keySelector, elementSelector, comparer)
//                   .Select(g => resultSelector(g.Key, g));
//        }

//        /// <summary>
//        /// Applies an accumulator function over a sequence.
//        /// </summary>

//        public static TSource Aggregate<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TSource, TSource> func)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(func, "func");

//            using (var e = source.GetEnumerator())
//            {
//                if (!e.MoveNext())
//                    throw new InvalidOperationException();

//                return e.Renumerable().Skip(1).Aggregate(e.Current, func);
//            }
//        }

//        /// <summary>
//        /// Applies an accumulator function over a sequence. The specified 
//        /// seed value is used as the initial accumulator value.
//        /// </summary>

//        public static TAccumulate Aggregate<TSource, TAccumulate>(
//            this IEnumerable<TSource> source,
//            TAccumulate seed,
//            Func<TAccumulate, TSource, TAccumulate> func)
//        {
//            return Aggregate(source, seed, func, r => r);
//        }

//        /// <summary>
//        /// Applies an accumulator function over a sequence. The specified 
//        /// seed value is used as the initial accumulator value, and the 
//        /// specified function is used to select the result value.
//        /// </summary>

//        public static TResult Aggregate<TSource, TAccumulate, TResult>(
//            this IEnumerable<TSource> source,
//            TAccumulate seed,
//            Func<TAccumulate, TSource, TAccumulate> func,
//            Func<TAccumulate, TResult> resultSelector)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(func, "func");
//            CheckNotNull(resultSelector, "resultSelector");

//            var result = seed;

//            foreach (var item in source)
//                result = func(result, item);

//            return resultSelector(result);
//        }

//        /// <summary>
//        /// Produces the set union of two sequences by using the default 
//        /// equality comparer.
//        /// </summary>

//        public static IEnumerable<TSource> Union<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second)
//        {
//            return Union(first, second, /* comparer */ null);
//        }

//        /// <summary>
//        /// Produces the set union of two sequences by using a specified 
//        /// <see cref="IEqualityComparer{T}" />.
//        /// </summary>

//        public static IEnumerable<TSource> Union<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second,
//            IEqualityComparer<TSource> comparer)
//        {
//            return first.Concat(second).Distinct(comparer);
//        }

//        /// <summary>
//        /// Returns the elements of the specified sequence or the type 
//        /// parameter's default value in a singleton collection if the 
//        /// sequence is empty.
//        /// </summary>

//        public static IEnumerable<TSource> DefaultIfEmpty<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            return source.DefaultIfEmpty(default(TSource));
//        }

//        /// <summary>
//        /// Returns the elements of the specified sequence or the specified 
//        /// value in a singleton collection if the sequence is empty.
//        /// </summary>

//        public static IEnumerable<TSource> DefaultIfEmpty<TSource>(
//            this IEnumerable<TSource> source,
//            TSource defaultValue)
//        {
//            CheckNotNull(source, "source");

//            return DefaultIfEmptyYield(source, defaultValue);
//        }

//        private static IEnumerable<TSource> DefaultIfEmptyYield<TSource>(
//            IEnumerable<TSource> source,
//            TSource defaultValue)
//        {
//            using (var e = source.GetEnumerator())
//            {
//                if (!e.MoveNext())
//                    yield return defaultValue;
//                else
//                    do { yield return e.Current; } while (e.MoveNext());
//            }
//        }

//        /// <summary>
//        /// Determines whether all elements of a sequence satisfy a condition.
//        /// </summary>

//        public static bool All<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(predicate, "predicate");

//            foreach (var item in source)
//                if (!predicate(item))
//                    return false;

//            return true;
//        }

//        /// <summary>
//        /// Determines whether a sequence contains any elements.
//        /// </summary>

//        public static bool Any<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            CheckNotNull(source, "source");

//            using (var e = source.GetEnumerator())
//                return e.MoveNext();
//        }

//        /// <summary>
//        /// Determines whether any element of a sequence satisfies a 
//        /// condition.
//        /// </summary>

//        public static bool Any<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, bool> predicate)
//        {
//            return source.Where(predicate).Any();
//        }

//        /// <summary>
//        /// Determines whether a sequence contains a specified element by 
//        /// using the default equality comparer.
//        /// </summary>

//        public static bool Contains<TSource>(
//            this IEnumerable<TSource> source,
//            TSource value)
//        {
//            return source.Contains(value, /* comparer */ null);
//        }

//        /// <summary>
//        /// Determines whether a sequence contains a specified element by 
//        /// using a specified <see cref="IEqualityComparer{T}" />.
//        /// </summary>

//        public static bool Contains<TSource>(
//            this IEnumerable<TSource> source,
//            TSource value,
//            IEqualityComparer<TSource> comparer)
//        {
//            CheckNotNull(source, "source");

//            if (comparer == null)
//            {
//                var collection = source as ICollection<TSource>;
//                if (collection != null)
//                    return collection.Contains(value);
//            }

//            comparer = comparer ?? EqualityComparer<TSource>.Default;
//            return source.Any(item => comparer.Equals(item, value));
//        }

//        /// <summary>
//        /// Determines whether two sequences are equal by comparing the 
//        /// elements by using the default equality comparer for their type.
//        /// </summary>

//        public static bool SequenceEqual<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second)
//        {
//            return first.SequenceEqual(second, /* comparer */ null);
//        }

//        /// <summary>
//        /// Determines whether two sequences are equal by comparing their 
//        /// elements by using a specified <see cref="IEqualityComparer{T}" />.
//        /// </summary>

//        public static bool SequenceEqual<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second,
//            IEqualityComparer<TSource> comparer)
//        {
//            CheckNotNull(first, "frist");
//            CheckNotNull(second, "second");

//            comparer = comparer ?? EqualityComparer<TSource>.Default;

//            using (IEnumerator<TSource> lhs = first.GetEnumerator(),
//                                        rhs = second.GetEnumerator())
//            {
//                do
//                {
//                    if (!lhs.MoveNext())
//                        return !rhs.MoveNext();

//                    if (!rhs.MoveNext())
//                        return false;
//                }
//                while (comparer.Equals(lhs.Current, rhs.Current));
//            }

//            return false;
//        }

//        /// <summary>
//        /// Base implementation for Min/Max operator.
//        /// </summary>

//        private static TSource MinMaxImpl<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TSource, bool> lesser)
//        {
//            CheckNotNull(source, "source");
//            Debug.Assert(lesser != null);

//            return source.Aggregate((a, item) => lesser(a, item) ? a : item);
//        }

//        /// <summary>
//        /// Base implementation for Min/Max operator for nullable types.
//        /// </summary>

//        private static TSource? MinMaxImpl<TSource>(
//            this IEnumerable<TSource?> source,
//            TSource? seed, Func<TSource?, TSource?, bool> lesser) where TSource : struct
//        {
//            CheckNotNull(source, "source");
//            Debug.Assert(lesser != null);

//            return source.Aggregate(seed, (a, item) => lesser(a, item) ? a : item);
//            //  == MinMaxImpl(Repeat<TSource?>(null, 1).Concat(source), lesser);
//        }

//        /// <summary>
//        /// Returns the minimum value in a generic sequence.
//        /// </summary>

//        public static TSource Min<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            var comparer = Comparer<TSource>.Default;
//            return source.MinMaxImpl((x, y) => comparer.Compare(x, y) < 0);
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a generic 
//        /// sequence and returns the minimum resulting value.
//        /// </summary>

//        public static TResult Min<TSource, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TResult> selector)
//        {
//            return source.Select(selector).Min();
//        }

//        /// <summary>
//        /// Returns the maximum value in a generic sequence.
//        /// </summary>

//        public static TSource Max<TSource>(
//            this IEnumerable<TSource> source)
//        {
//            var comparer = Comparer<TSource>.Default;
//            return source.MinMaxImpl((x, y) => comparer.Compare(x, y) > 0);
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a generic 
//        /// sequence and returns the maximum resulting value.
//        /// </summary>

//        public static TResult Max<TSource, TResult>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TResult> selector)
//        {
//            return source.Select(selector).Max();
//        }

//        /// <summary>
//        /// Makes an enumerator seen as enumerable once more.
//        /// </summary>
//        /// <remarks>
//        /// The supplied enumerator must have been started. The first element
//        /// returned is the element the enumerator was on when passed in.
//        /// DO NOT use this method if the caller must be a generator. It is
//        /// mostly safe among aggregate operations.
//        /// </remarks>

//        private static IEnumerable<T> Renumerable<T>(this IEnumerator<T> e)
//        {
//            Debug.Assert(e != null);

//            do { yield return e.Current; } while (e.MoveNext());
//        }

//        /// <summary>
//        /// Sorts the elements of a sequence in ascending order according to a key.
//        /// </summary>

//        public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector)
//        {
//            return source.OrderBy(keySelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Sorts the elements of a sequence in ascending order by using a 
//        /// specified comparer.
//        /// </summary>

//        public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            IComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(keySelector, "keySelector");

//            return new OrderedEnumerable<TSource, TKey>(source, keySelector, comparer, /* descending */ false);
//        }

//        /// <summary>
//        /// Sorts the elements of a sequence in descending order according to a key.
//        /// </summary>

//        public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector)
//        {
//            return source.OrderByDescending(keySelector, /* comparer */ null);
//        }

//        /// <summary>
//        ///  Sorts the elements of a sequence in descending order by using a 
//        /// specified comparer. 
//        /// </summary>

//        public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            IComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(source, "keySelector");

//            return new OrderedEnumerable<TSource, TKey>(source, keySelector, comparer, /* descending */ true);
//        }

//        /// <summary>
//        /// Performs a subsequent ordering of the elements in a sequence in 
//        /// ascending order according to a key.
//        /// </summary>

//        public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey>(
//            this IOrderedEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector)
//        {
//            return source.ThenBy(keySelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Performs a subsequent ordering of the elements in a sequence in 
//        /// ascending order by using a specified comparer.
//        /// </summary>

//        public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey>(
//            this IOrderedEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            IComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");

//            return source.CreateOrderedEnumerable(keySelector, comparer, /* descending */ false);
//        }

//        /// <summary>
//        /// Performs a subsequent ordering of the elements in a sequence in 
//        /// descending order, according to a key.
//        /// </summary>

//        public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey>(
//            this IOrderedEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector)
//        {
//            return source.ThenByDescending(keySelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Performs a subsequent ordering of the elements in a sequence in 
//        /// descending order by using a specified comparer.
//        /// </summary>

//        public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey>(
//            this IOrderedEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            IComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");

//            return source.CreateOrderedEnumerable(keySelector, comparer, /* descending */ true);
//        }

//        /// <summary>
//        /// Base implementation for Intersect and Except operators.
//        /// </summary>

//        private static IEnumerable<TSource> IntersectExceptImpl<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second,
//            IEqualityComparer<TSource> comparer,
//            bool flag)
//        {
//            CheckNotNull(first, "first");
//            CheckNotNull(second, "second");

//            var keys = new List<TSource>();
//            var flags = new Dictionary<TSource, bool>(comparer);

//            foreach (var item in first.Where(k => !flags.ContainsKey(k)))
//            {
//                flags.Add(item, !flag);
//                keys.Add(item);
//            }

//            foreach (var item in second.Where(flags.ContainsKey))
//                flags[item] = flag;

//            //
//            // As per docs, "the marked elements are yielded in the order in 
//            // which they were collected.
//            //

//            return keys.Where(item => flags[item]);
//        }

//        /// <summary>
//        /// Produces the set intersection of two sequences by using the 
//        /// default equality comparer to compare values.
//        /// </summary>

//        public static IEnumerable<TSource> Intersect<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second)
//        {
//            return first.Intersect(second, /* comparer */ null);
//        }

//        /// <summary>
//        /// Produces the set intersection of two sequences by using the 
//        /// specified <see cref="IEqualityComparer{T}" /> to compare values.
//        /// </summary>

//        public static IEnumerable<TSource> Intersect<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second,
//            IEqualityComparer<TSource> comparer)
//        {
//            return IntersectExceptImpl(first, second, comparer, /* flag */ true);
//        }

//        /// <summary>
//        /// Produces the set difference of two sequences by using the 
//        /// default equality comparer to compare values.
//        /// </summary>

//        public static IEnumerable<TSource> Except<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second)
//        {
//            return first.Except(second, /* comparer */ null);
//        }

//        /// <summary>
//        /// Produces the set difference of two sequences by using the 
//        /// specified <see cref="IEqualityComparer{T}" /> to compare values.
//        /// </summary>

//        public static IEnumerable<TSource> Except<TSource>(
//            this IEnumerable<TSource> first,
//            IEnumerable<TSource> second,
//            IEqualityComparer<TSource> comparer)
//        {
//            return IntersectExceptImpl(first, second, comparer, /* flag */ false);
//        }

//        /// <summary>
//        /// Creates a <see cref="Dictionary{TKey,TValue}" /> from an 
//        /// <see cref="IEnumerable{T}" /> according to a specified key 
//        /// selector function.
//        /// </summary>

//        public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector)
//        {
//            return source.ToDictionary(keySelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Creates a <see cref="Dictionary{TKey,TValue}" /> from an 
//        /// <see cref="IEnumerable{T}" /> according to a specified key 
//        /// selector function and key comparer.
//        /// </summary>

//        public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            return source.ToDictionary(keySelector, e => e);
//        }

//        /// <summary>
//        /// Creates a <see cref="Dictionary{TKey,TValue}" /> from an 
//        /// <see cref="IEnumerable{T}" /> according to specified key 
//        /// selector and element selector functions.
//        /// </summary>

//        public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TSource, TElement> elementSelector)
//        {
//            return source.ToDictionary(keySelector, elementSelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Creates a <see cref="Dictionary{TKey,TValue}" /> from an 
//        /// <see cref="IEnumerable{T}" /> according to a specified key 
//        /// selector function, a comparer, and an element selector function.
//        /// </summary>

//        public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement>(
//            this IEnumerable<TSource> source,
//            Func<TSource, TKey> keySelector,
//            Func<TSource, TElement> elementSelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            CheckNotNull(source, "source");
//            CheckNotNull(keySelector, "keySelector");
//            CheckNotNull(elementSelector, "elementSelector");

//            var dict = new Dictionary<TKey, TElement>(comparer);

//            foreach (var item in source)
//            {
//                //
//                // ToDictionary is meant to throw ArgumentNullException if
//                // keySelector produces a key that is null and 
//                // Argument exception if keySelector produces duplicate keys 
//                // for two elements. Incidentally, the doucmentation for
//                // IDictionary<TKey, TValue>.Add says that the Add method
//                // throws the same exceptions under the same circumstances
//                // so we don't need to do any additional checking or work
//                // here and let the Add implementation do all the heavy
//                // lifting.
//                //

//                dict.Add(keySelector(item), elementSelector(item));
//            }

//            return dict;
//        }

//        /// <summary>
//        /// Correlates the elements of two sequences based on matching keys. 
//        /// The default equality comparer is used to compare keys.
//        /// </summary>

//        public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(
//            this IEnumerable<TOuter> outer,
//            IEnumerable<TInner> inner,
//            Func<TOuter, TKey> outerKeySelector,
//            Func<TInner, TKey> innerKeySelector,
//            Func<TOuter, TInner, TResult> resultSelector)
//        {
//            return outer.Join(inner, outerKeySelector, innerKeySelector, resultSelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Correlates the elements of two sequences based on matching keys. 
//        /// The default equality comparer is used to compare keys. A 
//        /// specified <see cref="IEqualityComparer{T}" /> is used to compare keys.
//        /// </summary>

//        public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(
//            this IEnumerable<TOuter> outer,
//            IEnumerable<TInner> inner,
//            Func<TOuter, TKey> outerKeySelector,
//            Func<TInner, TKey> innerKeySelector,
//            Func<TOuter, TInner, TResult> resultSelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            CheckNotNull(outer, "outer");
//            CheckNotNull(inner, "inner");
//            CheckNotNull(outerKeySelector, "outerKeySelector");
//            CheckNotNull(innerKeySelector, "innerKeySelector");
//            CheckNotNull(resultSelector, "resultSelector");

//            var lookup = inner.ToLookup(innerKeySelector, comparer);

//            return
//                from o in outer
//                from i in lookup[outerKeySelector(o)]
//                select resultSelector(o, i);
//        }

//        /// <summary>
//        /// Correlates the elements of two sequences based on equality of 
//        /// keys and groups the results. The default equality comparer is 
//        /// used to compare keys.
//        /// </summary>

//        public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
//            this IEnumerable<TOuter> outer,
//            IEnumerable<TInner> inner,
//            Func<TOuter, TKey> outerKeySelector,
//            Func<TInner, TKey> innerKeySelector,
//            Func<TOuter, IEnumerable<TInner>, TResult> resultSelector)
//        {
//            return outer.GroupJoin(inner, outerKeySelector, innerKeySelector, resultSelector, /* comparer */ null);
//        }

//        /// <summary>
//        /// Correlates the elements of two sequences based on equality of 
//        /// keys and groups the results. The default equality comparer is 
//        /// used to compare keys. A specified <see cref="IEqualityComparer{T}" /> 
//        /// is used to compare keys.
//        /// </summary>

//        public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
//            this IEnumerable<TOuter> outer,
//            IEnumerable<TInner> inner,
//            Func<TOuter, TKey> outerKeySelector,
//            Func<TInner, TKey> innerKeySelector,
//            Func<TOuter, IEnumerable<TInner>, TResult> resultSelector,
//            IEqualityComparer<TKey> comparer)
//        {
//            CheckNotNull(outer, "outer");
//            CheckNotNull(inner, "inner");
//            CheckNotNull(outerKeySelector, "outerKeySelector");
//            CheckNotNull(innerKeySelector, "innerKeySelector");
//            CheckNotNull(resultSelector, "resultSelector");

//            var lookup = inner.ToLookup(innerKeySelector, comparer);
//            return outer.Select(o => resultSelector(o, lookup[outerKeySelector(o)]));
//        }

//        [DebuggerStepThrough]
//        private static void CheckNotNull<T>(T value, string name) where T : class
//        {
//            if (value == null)
//                throw new ArgumentNullException(name);
//        }

//        private static class Sequence<T>
//        {
//            public static readonly IEnumerable<T> Empty = new T[0];
//        }

//        private sealed class Grouping<K, V> : List<V>, IGrouping<K, V>
//        {
//            internal Grouping(K key)
//            {
//                Key = key;
//            }

//            public K Key { get; private set; }
//        }
//    }
//}

//// $Id: Enumerable.g.cs 215 2009-10-03 13:31:49Z azizatif $

//namespace System.Linq
//{
//    #region Imports

//    using System;
//    using System.Collections.Generic;

//    #endregion

//    // This partial implementation was template-generated:
//    // Sat, 03 Oct 2009 09:42:39 GMT

//    partial class Enumerable
//    {
//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Int32" /> values.
//        /// </summary>

//        public static int Sum(
//            this IEnumerable<int> source)
//        {
//            CheckNotNull(source, "source");

//            int sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + num);

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Int32" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static int Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Int32" /> values.
//        /// </summary>

//        public static double Average(
//            this IEnumerable<int> source)
//        {
//            CheckNotNull(source, "source");

//            long sum = 0;
//            long count = 0;

//            foreach (var num in source)
//                checked
//                {
//                    sum += (int)num;
//                    count++;
//                }

//            if (count == 0)
//                throw new InvalidOperationException();

//            return (double)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Int32" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static double Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int> selector)
//        {
//            return source.Select(selector).Average();
//        }


//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Int32" /> values.
//        /// </summary>

//        public static int? Sum(
//            this IEnumerable<int?> source)
//        {
//            CheckNotNull(source, "source");

//            int sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + (num ?? 0));

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Int32" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static int? Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int?> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Int32" /> values.
//        /// </summary>

//        public static double? Average(
//            this IEnumerable<int?> source)
//        {
//            CheckNotNull(source, "source");

//            long sum = 0;
//            long count = 0;

//            foreach (var num in source.Where(n => n != null))
//                checked
//                {
//                    sum += (int)num;
//                    count++;
//                }

//            if (count == 0)
//                return null;

//            return (double?)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Int32" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static double? Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int?> selector)
//        {
//            return source.Select(selector).Average();
//        }

//        /// <summary>
//        /// Returns the minimum value in a sequence of nullable 
//        /// <see cref="System.Int32" /> values.
//        /// </summary>

//        public static int? Min(
//            this IEnumerable<int?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null), null, (min, x) => min < x);
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the minimum nullable <see cref="System.Int32" /> value.
//        /// </summary>

//        public static int? Min<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int?> selector)
//        {
//            return source.Select(selector).Min();
//        }

//        /// <summary>
//        /// Returns the maximum value in a sequence of nullable 
//        /// <see cref="System.Int32" /> values.
//        /// </summary>

//        public static int? Max(
//            this IEnumerable<int?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null),
//                null, (max, x) => x == null || (max != null && x.Value < max.Value));
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the maximum nullable <see cref="System.Int32" /> value.
//        /// </summary>

//        public static int? Max<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, int?> selector)
//        {
//            return source.Select(selector).Max();
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Int64" /> values.
//        /// </summary>

//        public static long Sum(
//            this IEnumerable<long> source)
//        {
//            CheckNotNull(source, "source");

//            long sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + num);

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Int64" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static long Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, long> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Int64" /> values.
//        /// </summary>

//        public static double Average(
//            this IEnumerable<long> source)
//        {
//            CheckNotNull(source, "source");

//            long sum = 0;
//            long count = 0;

//            foreach (var num in source)
//                checked
//                {
//                    sum += (long)num;
//                    count++;
//                }

//            if (count == 0)
//                throw new InvalidOperationException();

//            return (double)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Int64" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static double Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, long> selector)
//        {
//            return source.Select(selector).Average();
//        }


//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Int64" /> values.
//        /// </summary>

//        public static long? Sum(
//            this IEnumerable<long?> source)
//        {
//            CheckNotNull(source, "source");

//            long sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + (num ?? 0));

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Int64" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static long? Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, long?> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Int64" /> values.
//        /// </summary>

//        public static double? Average(
//            this IEnumerable<long?> source)
//        {
//            CheckNotNull(source, "source");

//            long sum = 0;
//            long count = 0;

//            foreach (var num in source.Where(n => n != null))
//                checked
//                {
//                    sum += (long)num;
//                    count++;
//                }

//            if (count == 0)
//                return null;

//            return (double?)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Int64" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static double? Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, long?> selector)
//        {
//            return source.Select(selector).Average();
//        }

//        /// <summary>
//        /// Returns the minimum value in a sequence of nullable 
//        /// <see cref="System.Int64" /> values.
//        /// </summary>

//        public static long? Min(
//            this IEnumerable<long?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null), null, (min, x) => min < x);
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the minimum nullable <see cref="System.Int64" /> value.
//        /// </summary>

//        public static long? Min<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, long?> selector)
//        {
//            return source.Select(selector).Min();
//        }

//        /// <summary>
//        /// Returns the maximum value in a sequence of nullable 
//        /// <see cref="System.Int64" /> values.
//        /// </summary>

//        public static long? Max(
//            this IEnumerable<long?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null),
//                null, (max, x) => x == null || (max != null && x.Value < max.Value));
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the maximum nullable <see cref="System.Int64" /> value.
//        /// </summary>

//        public static long? Max<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, long?> selector)
//        {
//            return source.Select(selector).Max();
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Single" /> values.
//        /// </summary>

//        public static float Sum(
//            this IEnumerable<float> source)
//        {
//            CheckNotNull(source, "source");

//            float sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + num);

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Single" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static float Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, float> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Single" /> values.
//        /// </summary>

//        public static float Average(
//            this IEnumerable<float> source)
//        {
//            CheckNotNull(source, "source");

//            float sum = 0;
//            long count = 0;

//            foreach (var num in source)
//                checked
//                {
//                    sum += (float)num;
//                    count++;
//                }

//            if (count == 0)
//                throw new InvalidOperationException();

//            return (float)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Single" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static float Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, float> selector)
//        {
//            return source.Select(selector).Average();
//        }


//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Single" /> values.
//        /// </summary>

//        public static float? Sum(
//            this IEnumerable<float?> source)
//        {
//            CheckNotNull(source, "source");

//            float sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + (num ?? 0));

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Single" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static float? Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, float?> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Single" /> values.
//        /// </summary>

//        public static float? Average(
//            this IEnumerable<float?> source)
//        {
//            CheckNotNull(source, "source");

//            float sum = 0;
//            long count = 0;

//            foreach (var num in source.Where(n => n != null))
//                checked
//                {
//                    sum += (float)num;
//                    count++;
//                }

//            if (count == 0)
//                return null;

//            return (float?)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Single" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static float? Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, float?> selector)
//        {
//            return source.Select(selector).Average();
//        }

//        /// <summary>
//        /// Returns the minimum value in a sequence of nullable 
//        /// <see cref="System.Single" /> values.
//        /// </summary>

//        public static float? Min(
//            this IEnumerable<float?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null), null, (min, x) => min < x);
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the minimum nullable <see cref="System.Single" /> value.
//        /// </summary>

//        public static float? Min<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, float?> selector)
//        {
//            return source.Select(selector).Min();
//        }

//        /// <summary>
//        /// Returns the maximum value in a sequence of nullable 
//        /// <see cref="System.Single" /> values.
//        /// </summary>

//        public static float? Max(
//            this IEnumerable<float?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null),
//                null, (max, x) => x == null || (max != null && x.Value < max.Value));
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the maximum nullable <see cref="System.Single" /> value.
//        /// </summary>

//        public static float? Max<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, float?> selector)
//        {
//            return source.Select(selector).Max();
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Double" /> values.
//        /// </summary>

//        public static double Sum(
//            this IEnumerable<double> source)
//        {
//            CheckNotNull(source, "source");

//            double sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + num);

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Double" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static double Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, double> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Double" /> values.
//        /// </summary>

//        public static double Average(
//            this IEnumerable<double> source)
//        {
//            CheckNotNull(source, "source");

//            double sum = 0;
//            long count = 0;

//            foreach (var num in source)
//                checked
//                {
//                    sum += (double)num;
//                    count++;
//                }

//            if (count == 0)
//                throw new InvalidOperationException();

//            return (double)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Double" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static double Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, double> selector)
//        {
//            return source.Select(selector).Average();
//        }


//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Double" /> values.
//        /// </summary>

//        public static double? Sum(
//            this IEnumerable<double?> source)
//        {
//            CheckNotNull(source, "source");

//            double sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + (num ?? 0));

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Double" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static double? Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, double?> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Double" /> values.
//        /// </summary>

//        public static double? Average(
//            this IEnumerable<double?> source)
//        {
//            CheckNotNull(source, "source");

//            double sum = 0;
//            long count = 0;

//            foreach (var num in source.Where(n => n != null))
//                checked
//                {
//                    sum += (double)num;
//                    count++;
//                }

//            if (count == 0)
//                return null;

//            return (double?)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Double" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static double? Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, double?> selector)
//        {
//            return source.Select(selector).Average();
//        }

//        /// <summary>
//        /// Returns the minimum value in a sequence of nullable 
//        /// <see cref="System.Double" /> values.
//        /// </summary>

//        public static double? Min(
//            this IEnumerable<double?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null), null, (min, x) => min < x);
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the minimum nullable <see cref="System.Double" /> value.
//        /// </summary>

//        public static double? Min<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, double?> selector)
//        {
//            return source.Select(selector).Min();
//        }

//        /// <summary>
//        /// Returns the maximum value in a sequence of nullable 
//        /// <see cref="System.Double" /> values.
//        /// </summary>

//        public static double? Max(
//            this IEnumerable<double?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null),
//                null, (max, x) => x == null || (max != null && x.Value < max.Value));
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the maximum nullable <see cref="System.Double" /> value.
//        /// </summary>

//        public static double? Max<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, double?> selector)
//        {
//            return source.Select(selector).Max();
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Decimal" /> values.
//        /// </summary>

//        public static decimal Sum(
//            this IEnumerable<decimal> source)
//        {
//            CheckNotNull(source, "source");

//            decimal sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + num);

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of nullable <see cref="System.Decimal" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static decimal Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, decimal> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Decimal" /> values.
//        /// </summary>

//        public static decimal Average(
//            this IEnumerable<decimal> source)
//        {
//            CheckNotNull(source, "source");

//            decimal sum = 0;
//            long count = 0;

//            foreach (var num in source)
//                checked
//                {
//                    sum += (decimal)num;
//                    count++;
//                }

//            if (count == 0)
//                throw new InvalidOperationException();

//            return (decimal)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of nullable <see cref="System.Decimal" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static decimal Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, decimal> selector)
//        {
//            return source.Select(selector).Average();
//        }


//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Decimal" /> values.
//        /// </summary>

//        public static decimal? Sum(
//            this IEnumerable<decimal?> source)
//        {
//            CheckNotNull(source, "source");

//            decimal sum = 0;
//            foreach (var num in source)
//                sum = checked(sum + (num ?? 0));

//            return sum;
//        }

//        /// <summary>
//        /// Computes the sum of a sequence of <see cref="System.Decimal" /> 
//        /// values that are obtained by invoking a transform function on 
//        /// each element of the input sequence.
//        /// </summary>

//        public static decimal? Sum<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, decimal?> selector)
//        {
//            return source.Select(selector).Sum();
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Decimal" /> values.
//        /// </summary>

//        public static decimal? Average(
//            this IEnumerable<decimal?> source)
//        {
//            CheckNotNull(source, "source");

//            decimal sum = 0;
//            long count = 0;

//            foreach (var num in source.Where(n => n != null))
//                checked
//                {
//                    sum += (decimal)num;
//                    count++;
//                }

//            if (count == 0)
//                return null;

//            return (decimal?)sum / count;
//        }

//        /// <summary>
//        /// Computes the average of a sequence of <see cref="System.Decimal" /> values 
//        /// that are obtained by invoking a transform function on each 
//        /// element of the input sequence.
//        /// </summary>

//        public static decimal? Average<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, decimal?> selector)
//        {
//            return source.Select(selector).Average();
//        }

//        /// <summary>
//        /// Returns the minimum value in a sequence of nullable 
//        /// <see cref="System.Decimal" /> values.
//        /// </summary>

//        public static decimal? Min(
//            this IEnumerable<decimal?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null), null, (min, x) => min < x);
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the minimum nullable <see cref="System.Decimal" /> value.
//        /// </summary>

//        public static decimal? Min<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, decimal?> selector)
//        {
//            return source.Select(selector).Min();
//        }

//        /// <summary>
//        /// Returns the maximum value in a sequence of nullable 
//        /// <see cref="System.Decimal" /> values.
//        /// </summary>

//        public static decimal? Max(
//            this IEnumerable<decimal?> source)
//        {
//            CheckNotNull(source, "source");

//            return MinMaxImpl(source.Where(x => x != null),
//                null, (max, x) => x == null || (max != null && x.Value < max.Value));
//        }

//        /// <summary>
//        /// Invokes a transform function on each element of a sequence and 
//        /// returns the maximum nullable <see cref="System.Decimal" /> value.
//        /// </summary>

//        public static decimal? Max<TSource>(
//            this IEnumerable<TSource> source,
//            Func<TSource, decimal?> selector)
//        {
//            return source.Select(selector).Max();
//        }
//    }
//}

//namespace System.Runtime.CompilerServices
//{
//    /// <remarks>
//    /// This attribute allows us to define extension methods without 
//    /// requiring .NET Framework 3.5. For more information, see the section,
//    /// <a href="http://msdn.microsoft.com/en-us/magazine/cc163317.aspx#S7">Extension Methods in .NET Framework 2.0 Apps</a>,
//    /// of <a href="http://msdn.microsoft.com/en-us/magazine/cc163317.aspx">Basic Instincts: Extension Methods</a>
//    /// column in <a href="http://msdn.microsoft.com/msdnmag/">MSDN Magazine</a>, 
//    /// issue <a href="http://msdn.microsoft.com/en-us/magazine/cc135410.aspx">Nov 2007</a>.
//    /// </remarks>

//    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class | AttributeTargets.Assembly)]
//    internal sealed class ExtensionAttribute : Attribute { }
//}

//// $Id: Func.cs 224 2009-10-04 07:13:08Z azizatif $

//namespace System
//{
//#if LINQBRIDGE_LIB
//    public delegate TResult Func<TResult>();
//    public delegate TResult Func<T, TResult>(T a);
//    public delegate TResult Func<T1, T2, TResult>(T1 arg1, T2 arg2);
//    public delegate TResult Func<T1, T2, T3, TResult>(T1 arg1, T2 arg2, T3 arg3);
//    public delegate TResult Func<T1, T2, T3, T4, TResult>(T1 arg1, T2 arg2, T3 arg3, T4 arg4);
//#else
//    delegate TResult Func<TResult>();
//    delegate TResult Func<T, TResult>(T a);
//    delegate TResult Func<T1, T2, TResult>(T1 arg1, T2 arg2);
//    delegate TResult Func<T1, T2, T3, TResult>(T1 arg1, T2 arg2, T3 arg3);
//    delegate TResult Func<T1, T2, T3, T4, TResult>(T1 arg1, T2 arg2, T3 arg3, T4 arg4);
//#endif
//}

//// $Id: IGrouping.cs 225 2009-10-04 07:16:14Z azizatif $

//namespace System.Linq
//{
//    #region Imports

//    using System.Collections.Generic;

//    #endregion

//    /// <summary>
//    /// Represents a collection of objects that have a common key.
//    /// </summary>

//    partial interface IGrouping<TKey, TElement> : IEnumerable<TElement>
//    {
//        /// <summary>
//        /// Gets the key of the <see cref="IGrouping{TKey,TElement}" />.
//        /// </summary>

//        TKey Key { get; }
//    }
//}

//// $Id: ILookup.cs 224 2009-10-04 07:13:08Z azizatif $

//namespace System.Linq
//{
//    using System.Collections.Generic;

//    /// <summary>
//    /// Defines an indexer, size property, and Boolean search method for 
//    /// data structures that map keys to <see cref="IEnumerable{T}"/> 
//    /// sequences of values.
//    /// </summary>

//    partial interface ILookup<TKey, TElement> : IEnumerable<IGrouping<TKey, TElement>>
//    {
//        bool Contains(TKey key);
//        int Count { get; }
//        IEnumerable<TElement> this[TKey key] { get; }
//    }
//}

//// $Id: IOrderedEnumerable.cs 224 2009-10-04 07:13:08Z azizatif $

//namespace System.Linq
//{
//    using System.Collections.Generic;

//    /// <summary>
//    /// Represents a sorted sequence.
//    /// </summary>

//    partial interface IOrderedEnumerable<TElement> : IEnumerable<TElement>
//    {
//        /// <summary>
//        /// Performs a subsequent ordering on the elements of an 
//        /// <see cref="IOrderedEnumerable{T}"/> according to a key.
//        /// </summary>

//        IOrderedEnumerable<TElement> CreateOrderedEnumerable<TKey>(
//            Func<TElement, TKey> keySelector, IComparer<TKey> comparer, bool descending);
//    }
//}

//// $Id: Lookup.cs 224 2009-10-04 07:13:08Z azizatif $

//namespace System.Linq
//{
//    #region Imports

//    using System;
//    using System.Collections;
//    using System.Collections.Generic;
//    using IEnumerable = System.Collections.IEnumerable;

//    #endregion

//    /// <summary>
//    /// Represents a collection of keys each mapped to one or more values.
//    /// </summary>

//    internal sealed class Lookup<TKey, TElement> : ILookup<TKey, TElement>
//    {
//        private readonly Dictionary<TKey, IGrouping<TKey, TElement>> _map;

//        internal Lookup(IEqualityComparer<TKey> comparer)
//        {
//            _map = new Dictionary<TKey, IGrouping<TKey, TElement>>(comparer);
//        }

//        internal void Add(IGrouping<TKey, TElement> item)
//        {
//            _map.Add(item.Key, item);
//        }

//        internal IEnumerable<TElement> Find(TKey key)
//        {
//            IGrouping<TKey, TElement> grouping;
//            return _map.TryGetValue(key, out grouping) ? grouping : null;
//        }

//        /// <summary>
//        /// Gets the number of key/value collection pairs in the <see cref="Lookup{TKey,TElement}" />.
//        /// </summary>

//        public int Count
//        {
//            get { return _map.Count; }
//        }

//        /// <summary>
//        /// Gets the collection of values indexed by the specified key.
//        /// </summary>

//        public IEnumerable<TElement> this[TKey key]
//        {
//            get
//            {
//                IGrouping<TKey, TElement> result;
//                return _map.TryGetValue(key, out result) ? result : Enumerable.Empty<TElement>();
//            }
//        }

//        /// <summary>
//        /// Determines whether a specified key is in the <see cref="Lookup{TKey,TElement}" />.
//        /// </summary>

//        public bool Contains(TKey key)
//        {
//            return _map.ContainsKey(key);
//        }

//        /// <summary>
//        /// Applies a transform function to each key and its associated 
//        /// values and returns the results.
//        /// </summary>

//        public IEnumerable<TResult> ApplyResultSelector<TResult>(
//            Func<TKey, IEnumerable<TElement>, TResult> resultSelector)
//        {
//            if (resultSelector == null)
//                throw new ArgumentNullException("resultSelector");

//            foreach (var pair in _map)
//                yield return resultSelector(pair.Key, pair.Value);
//        }

//        /// <summary>
//        /// Returns a generic enumerator that iterates through the <see cref="Lookup{TKey,TElement}" />.
//        /// </summary>

//        public IEnumerator<IGrouping<TKey, TElement>> GetEnumerator()
//        {
//            return _map.Values.GetEnumerator();
//        }

//        IEnumerator IEnumerable.GetEnumerator()
//        {
//            return GetEnumerator();
//        }
//    }
//}

//// $Id: OrderedEnumerable.cs 237 2010-01-31 12:20:24Z azizatif $

//namespace LinqBridge
//{
//    #region Imports

//    using System;
//    using System.Collections;
//    using System.Collections.Generic;
//    using System.Linq;

//    #endregion

//    internal sealed class OrderedEnumerable<T, K> : IOrderedEnumerable<T>
//    {
//        private readonly IEnumerable<T> _source;
//        private readonly List<Comparison<T>> _comparisons;

//        public OrderedEnumerable(IEnumerable<T> source,
//            Func<T, K> keySelector, IComparer<K> comparer, bool descending) :
//            this(source, null, keySelector, comparer, descending)
//        { }

//        private OrderedEnumerable(IEnumerable<T> source, List<Comparison<T>> comparisons,
//            Func<T, K> keySelector, IComparer<K> comparer, bool descending)
//        {
//            if (source == null) throw new ArgumentNullException("source");
//            if (keySelector == null) throw new ArgumentNullException("keySelector");

//            _source = source;

//            comparer = comparer ?? Comparer<K>.Default;

//            if (comparisons == null)
//                comparisons = new List<Comparison<T>>(/* capacity */ 4);

//            comparisons.Add((x, y)
//                => (descending ? -1 : 1) * comparer.Compare(keySelector(x), keySelector(y)));

//            _comparisons = comparisons;
//        }

//        public IOrderedEnumerable<T> CreateOrderedEnumerable<KK>(
//            Func<T, KK> keySelector, IComparer<KK> comparer, bool descending)
//        {
//            return new OrderedEnumerable<T, KK>(_source, _comparisons, keySelector, comparer, descending);
//        }

//        public IEnumerator<T> GetEnumerator()
//        {
//            //
//            // We sort using List<T>.Sort, but docs say that it performs an 
//            // unstable sort. LINQ, on the other hand, says OrderBy performs 
//            // a stable sort. So convert the source sequence into a sequence 
//            // of tuples where the second element tags the position of the 
//            // element from the source sequence (First). The position is 
//            // then used as a tie breaker when all keys compare equal,
//            // thus making the sort stable.
//            //

//            var list = _source.Select(new Func<T, int, Tuple<T, int>>(TagPosition)).ToList();

//            list.Sort((x, y) =>
//            {
//                //
//                // Compare keys from left to right.
//                //

//                var comparisons = _comparisons;
//                for (var i = 0; i < comparisons.Count; i++)
//                {
//                    var result = comparisons[i](x.First, y.First);
//                    if (result != 0)
//                        return result;
//                }

//                //
//                // All keys compared equal so now break the tie by their
//                // position in the original sequence, making the sort stable.
//                //

//                return x.Second.CompareTo(y.Second);
//            });

//            return list.Select(new Func<Tuple<T, int>, T>(GetFirst)).GetEnumerator();

//        }

//        /// <remarks>
//        /// See <a href="http://code.google.com/p/linqbridge/issues/detail?id=11">issue #11</a>
//        /// for why this method is needed and cannot be expressed as a 
//        /// lambda at the call site.
//        /// </remarks>

//        private static Tuple<T, int> TagPosition(T e, int i)
//        {
//            return new Tuple<T, int>(e, i);
//        }

//        /// <remarks>
//        /// See <a href="http://code.google.com/p/linqbridge/issues/detail?id=11">issue #11</a>
//        /// for why this method is needed and cannot be expressed as a 
//        /// lambda at the call site.
//        /// </remarks>

//        private static T GetFirst(Tuple<T, int> pv)
//        {
//            return pv.First;
//        }

//        IEnumerator IEnumerable.GetEnumerator()
//        {
//            return GetEnumerator();
//        }
//    }
//}

//// $Id: Tuple.cs 215 2009-10-03 13:31:49Z azizatif $

//namespace LinqBridge
//{
//    #region Imports

//    using System;
//    using System.Collections.Generic;
//    using System.Text;

//    #endregion

//    [Serializable]
//    internal struct Tuple<TFirst, TSecond> : IEquatable<Tuple<TFirst, TSecond>>
//    {
//        public TFirst First { get; private set; }
//        public TSecond Second { get; private set; }

//        public Tuple(TFirst first, TSecond second) : this()
//        {
//            First = first;
//            Second = second;
//        }

//        public override bool Equals(object obj)
//        {
//            return obj != null
//                   && obj is Tuple<TFirst, TSecond>
//                   && base.Equals((Tuple<TFirst, TSecond>)obj);
//        }

//        public bool Equals(Tuple<TFirst, TSecond> other)
//        {
//            return EqualityComparer<TFirst>.Default.Equals(other.First, First)
//                   && EqualityComparer<TSecond>.Default.Equals(other.Second, Second);
//        }

//        public override int GetHashCode()
//        {
//            var num = 0x7a2f0b42;
//            num = (-1521134295 * num) + EqualityComparer<TFirst>.Default.GetHashCode(First);
//            return (-1521134295 * num) + EqualityComparer<TSecond>.Default.GetHashCode(Second);
//        }

//        public override string ToString()
//        {
//            return string.Format(@"{{ First = {0}, Second = {1} }}", First, Second);
//        }
//    }
//}

//// $Id: Action.cs 239 2010-02-05 23:26:23Z azizatif $

//namespace System
//{
//#if LINQBRIDGE_LIB
//    public delegate void Action();
//    public delegate void Action<T1, T2>(T1 arg1, T2 arg2);
//    public delegate void Action<T1, T2, T3>(T1 arg1, T2 arg2, T3 arg3);
//    public delegate void Action<T1, T2, T3, T4>(T1 arg1, T2 arg2, T3 arg3, T4 arg4);
//#else
//    delegate void Action();
//    delegate void Action<T1, T2>(T1 arg1, T2 arg2);
//    delegate void Action<T1, T2, T3>(T1 arg1, T2 arg2, T3 arg3);
//    delegate void Action<T1, T2, T3, T4>(T1 arg1, T2 arg2, T3 arg3, T4 arg4);
//#endif
//}